Kansas City Royals pitcher Nathan Karns says he will undergo surgery on July 19th to address thoracic outlet syndrome and expects to be ready by spring training. Video by John Sleezer and Rustin Dodd/The Kansas City Star John Sleezer and Rustin DoddThe Kansas City Star

Kansas City Royals pitcher Nathan Karns says he will undergo surgery on July 19th to address thoracic outlet syndrome and expects to be ready by spring training. Video by John Sleezer and Rustin Dodd/The Kansas City Star John Sleezer and Rustin DoddThe Kansas City Star

On May 19, Nathan Karns exited a start in Minnesota after just five innings, complaining of tightness on the top of his right forearm. The discomfort led to a stint on the 10-day disabled list.

The rehab process was expected to be short. Karns believed he would miss one or two starts. And then nothing went right.

On Wednesday, exactly two months later, Karns will undergo surgery to address thoracic outlet syndrome, a neurogenic condition caused by the compression of nerves near the neck and shoulder. The procedure will sideline Karns until spring training. It was expected after Royals manager Ned Yost told The Star last week that doctors suspected that Karns was suffering from thoracic outlet syndrome.

“My symptoms were kind of floating around, like every day was kind of a little bit different,” Karns said Saturday. “It took us a while to figure out what was going on.”

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Karns was acquired in an offseason deal that sent outfielder Jarrod Dyson, a pending free agent, to the Seattle Mariners. The Royals sought an addition to their starting rotation. General manager Dayton Moore said Saturday that the club’s pretrade medical evaluation of Karns offered no signs that he was suffering from thoracic outlet syndrome. Any hints, Moore said, would have likely quashed the trade.

Thoracic outlet syndrome can cause numbness in the fingers and soreness in the arm. The condition has become more common in baseball circles in recent years, especially among pitchers, though players and industry medical personnel say that is in part because there is more awareness of the issue.

In 2016, thoracic outlet — which is commonly known as TOS — felled Royals reliever Luke Hochevar, top prospect Kyle Zimmer and right-hander Dillon Gee. Former Royals pitcher Chris Young also suffered from the condition before a diagnosis helped extend his career.

The success rate for players returning from TOS is encouraging. Yet, it is still a serious procedure, one that includes the removal of a rib — a first rib resection — near the shoulder. Hochevar faced complications after his procedure that have prevented him from pitching this season. He is expected to return in 2018. Karns said his surgery will be performed by Gregory Pearl, a specialist based in Dallas.

“It’s something that you don’t take lightly,” Karns said. “But at the end of the day, you know there’s a good percentage that you come back the same, if not a little bit better.”

Karns, 29, is under club control through the 2020 season. In eight starts this season, he posted a 3.43 ERA with 49 strikeouts in 44 2/3 innings. He appeared to be an emerging member of the rotation. But the loss has thinned the depth of the unit.

The Royals entered the second half of the season with left-hander Travis Wood operating as their fifth starter. The club will likely search for upgrades as the July 31 trade deadline approaches. But the search could be dictated by the availability of arms in what appears to be a seller’s market.

“I’m content with this team,” Moore said. “I think this team is good enough to win. But you’re always looking for ways to improve your roster. We’ll look internally first, but at this time of the year, it makes perfect sense to evaluate the market.”